variables - In Javascript, <int-value> == "<int-value>" evaluates to true. Why is it so?

Question

If I do 0 == "0" it evaluates to true. Try,

if( -777 == "-777" ) alert("same");

alert happens.

And, it's also noticeable that true == "true" doesn't evaluate to true. Try,

if( false == "false" ) alert("same");

alert doesn't happen.

Why is it so?

Answer 1

The behavior of == is a bit lengthy, but clearly defined in the ecma-262 spec:

11.9.3 The Abstract Equality Comparison Algorithm

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

1. If Type(x) is different from Type(y), go to step 14.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is not Number, go to step 11.
5. If x is NaN, return false.
6. If y is NaN, return false.
7. If x is the same number value as y, return true.
8. If x is +0 and y is ?0, return true.
9. If x is ?0 and y is +0, return true.
10. Return false.
11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
12. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
14. If x is null and y is undefined, return true.
15. If x is undefined and y is null, return true.
16. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
17. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
18. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
20. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
21. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
22. Return false.

Step 16 applies to your former example:

   0 == "0"            // apply 16
≡  0 == toNumber("0")
≡  0 == 0              // apply 7
≡  true

And step 18, then step 16, apply to the latter:

   true == "true"            // apply 18
≡  toNumber(true) == "true"
≡  1 == "true"               // apply 16
≡  1 == toNumber("true")
≡  1 == NaN                  // apply 6
≡  false